Method of drying biodegradable polymers

ABSTRACT

The present invention relates to a process for reducing the solvent content from a biodegradable polymer in a composition or a preparation. The process comprises drying the biopolymer composition by submitting it to microwaves for a time to achieve removal of the solvent at desired levels.

TECHNICAL FIELD

The present invention relates to a drying or solution concentratingprocess. In particular, it relates to a process for drying a polarsolution of biodegradable polymer or biomass and a process forconcentrating a polar solution comprising a biodegradable polymer or abiomass.

BACKGROUND OF THE INVENTION

Biodegradable polymers such as polyhydroxyalkanoates (PHAs) are of greatinterest and have resulted in an interesting new source of commoditypolymers. Until recently, interest has been focused on PHAs productionbecause their physical properties are similar to those of polymersproduced from petrochemical sources (polyethylene, polypropylene) andunlike petrochemical based polymers, PHAs are both biocompatible andbiodegradable. The potential worldwide market for biodegradable andbiocompatible polymers is enormous. Polyhydroxyalkanoate (PHA) is achemical term for a specific class of polyesters family. PHAs arebiopolymers that are generally produced intracellularly by manymicroorganisms as an energy storage compound. The most common PHAbiopolymer is PHB (polyhydroxybutyrate). More than 90 different monomerscan be combined within this family to give materials with extremelydifferent interesting properties. PHAs have the advantage of beingbiodegradable and possess thermoplastic or elastomeric properties. PHAscan be found commercially in a variety of plastic articles, packaging(household products, personal care products and food packaging), papercoatings, medical implants and hygienic products (disposable diapers orpaper).

The use of PHA biodegradable polymers will be a viable alternative topetrochemical based polymers especially if the production and theextraction of PHAs are performed at low cost. A process for recoveringPHAs from a biological source material generally implies: providingbiomass containing PHA, lysing of the bacterial cell, extracting the PHAfrom the cell, separating the PHA from the other components of thebiological source material and recovering the PHA.

Some pre-treatments (used to weaken or to provoke lysis of cells) mayrequire dried biomass. Drying is usually performed under vacuum, on atray in a forced air tunnel, by spray drying or by freeze drying(lyophylization). Drying a cell suspension or PHA solution is anobligatory step that takes place at many other stages during the PHAproduction and extraction. Drying processes are generally requiredbefore the extraction of PHA from the biomass and after the recovery ofthe PHA from the PHA-enriched solvent.

U.S. Pat. No. 6,087,471 describes the extraction of PHA from the drybiomass with an effective PHA-poor solvent, wherein the dissolution stepis performed at a temperature above the boiling point of the PHA-poorsolvent and under pressure. The PHA-enriched solvent is separated fromthe residual insoluble biomass and then, the temperature of thePHA-enriched solvent is reduced, causing PHA precipitation. PHAprecipitate is recovered by filtration. PHA in form of powder isobtained by drying in a vacuum oven overnight at 50° C.

U.S. Pat. No. 5,821,299 describes the recovery of the PHA from the drybiomass (vacuum dried, spray dried, freeze dried or dried on a tray at30° C. in a forced air tunnel) by treating the biomass with a PHAsolvent and a marginal non-solvent for PHA. The insoluble biomass isremoved, thereby leaving a solution of PHA and a marginal non-solventfor PHA. The PHA solvent is removed and a suspension of precipitated PHAin a marginal non-solvent is obtained. PHA in the form of powder isobtained by filtration and drying.

U.S. Pat. No. 6,043,063 describes a method for PHA extraction from drybiomass by dissolving PHA to produce PHA-enriched solvent and residualbiomass materials, separating the residual biomass from the PHA-enrichedsolvent and recovering PHA polymer from the PHA-enriched solvent. Thepolymer is recovered by filtering, washing and drying in a vacuum ovenovernight at 45-50° C.

Drying the biomass recovered after the fermentation step could also bean interesting alternative to assure the long term conservation of thebiomass and to avoid microorganism proliferation. The importance of thedrying during the production of PHA explains the need for a simple andeconomical process for drying PHA solutions or biomass from thefermentation step to reduce the production costs of PHA biodegradablepolymers compared to synthetic petroleum based polymers.

Concentrating a cell suspension or a PHA biodegradable polymer solutionis often required before each new chemical treatment (ex: pre-treatmentof the biomass, PHA extraction with a solvent, washing with differentkinds of chemical agents and enzymatic treatments) during the PHAproduction process. Concentrating a cell suspension or PHA solution isusually performed by centrifugation.

European patent number A-0,015,669 describes the concentration range(from 5 to 15% by weight biomass solids) that must be obtained ratherafter the concentration of the cell suspension by centrifugation. Thecell suspension must be preferably concentrated by centrifugation priorto the extraction being processed (performed by combining a solvent withan aqueous suspension of the disrupted cells).

During the production of several kinds of biodegradable polymers, adrying or a concentrating step is required. For instance, polylacticacid (PLA) can be prepared by direct condensation of lactic acid or byring-opening polymerisation of the cyclic lactide dimer.

U.S. Pat. No. 5,142,023 describes a process for continuous production ofpolylactide polymer from lactic acid comprising water or solvent removalin order to concentrate the lactic acid followed by polymerisation. Theconcentrating step is performed by evaporating a substantial portion ofthe aqueous medium.

It could also be very interesting or necessary to dry a biodegradablepolymer before a thermoforming process to remove all traces of water orpolar solvents. Thus, it would be highly recommended to provide a dryingmethod that could be applied at any stage during the biodegradablepolymer production process. The present invention relates to a drying ora solution concentrating process using radiation heating techniques(infrared light, radio-frequency, microwaves and electrical resistanceused as generating heating source).

Microwave drying technique is an example of a radiation heatingtechnique that is already commercially used for the production ofsynthetic non biodegradable polymers. In some polymer making processes,it is necessary to remove solvents and/or water from the polymer. U.S.Pat. No. 4,055,001 describes the use of microwave drying processesduring the production of butyl rubber. Water and organic solvents areremoved from non-polar materials by passing said materials through apneumatic conveyor resonating cavity operating at a well definedmicrowave frequency. CN. Pat. No. 1,231,297 describes a method fordrying a high molecular polymer colloid in microwave heating equipment.The microwave heating time is less than 10 minutes, the energyconsumption is reduced and the production efficiency is increased.

Researchers from the LTEE (Laboratoire des technologies électrochimiqueset des électrotechnologies d'Hydro-Québec, Québec, Canada) have studiedradiation heating techniques (infrared light, radio-frequency,microwaves and electrical resistance as generating heating source) usedfor general drying applications. They found that radiation heatingtechniques reduce the greenhouse gases emission and increase the dryingefficiency compared to convection or conduction heating techniques.Radiation heating techniques have been recognised as being aninteresting alternative to conventional heating techniques.

By using radiation heating techniques instead of current dryingprocesses (heat treatment, vacuum dried, spray dried, freeze dried,dried on a tray in a forced air tunnel or lyophylization), the timerequired to completely dry a sample, the infrastructure and theconsumption of energy costs are considerably reduced. Radiation heatingtechniques are consequently a good alternative to conventional heatingprocesses.

Polyhydroxyalkanoates belong to the family of polyesters. When they arethermoformed, polyesters are very sensitive to hydrolysis, resulting inthe reduction of their molecular weight. Therefore, before thethermoforming process it is very important, even critical, to dry themin order to remove all traces of water (moistening percentage should bebelow 0.02%).

It would be highly desirable to have a new method of drying for dryingbiopolymers without degradation thereof.

SUMMARY OF THE INVENTION

One object of the present invention is to provide a process for dryingor concentrating a biodegradable polymer solution or biomass comprisingsubmitting the solution or biomass containing the polymer to microwavesfor a period of time sufficient to reduce solvent concentration from thesolution or biomass in proportions of between about 0.0001% to 100%. Thepolymer may be synthetic or natural, and it can be selected from thegroup consisting of polyester, polysaccharide, polyalcohol, polyacid, ora mixture or a copolymer thereof.

In accordance with the present invention there is provided a process fordrying or concentrating a polyester that can be selected from the groupconsisting of polyhydroxyalkanoate, polycaprolactone, polylactic acid,polyglycolic acid, poly(lactic-co-glycolic) acid, poly(succinic acid),or a mixture or a copolymer thereof, or polyalcohol that can be selectedfrom poly(vinyl alcohol), cellulose, or derivatives thereof.

Microwaves utilized in the process of the present invention can beselected between about 915 to 2450 MHz, and produce between about 100 to1500 Watts.

Another object of the present invention is to provide a process by whichthe drying or concentrating is performed with less than 5% degradationof a polymer in solution or biomass.

The solvent used to solubilize the polymers can be an aqueous medium, aswater for example, or polar organic solvent that can be selected fromthe group consisting of alcohol, amine, amide, halogenous, cyano,aldehyde, acid, cetone, ester, thiol, and sulfoxide.

Also, the process of the invention may be performed to concentrate ordry a polyhydroxyalkanoate that can be selected from the groupconsisting of poly-3-hydroxybutyrate, poly-3-hydroxyvalerate,poly-3-hydroxypentanoate, poly-3-hydroxyhexanoate,poly-3-hydroxyheptanoate; poly-3-hydroxyoctanoate,poly-3-hydroxynonanoate, poly-3-hydroxydecanoate,poly-3-hydroxydodecanoate, poly-4-hydroxybutyrate, and medium chainlength PHA, or a mixture or a copolymer thereof.

The process of the present invention allows for drying and/orconcentrating a biopolymer or biodegradable polymer in a solution or ina biomass by inducing no or a low degree of degradation to thebiopolymer. The degradation level may vary from 0 to 25%. Preferably thedegradation level is between 0 to 10%, and most preferably between 0 to2%.

For the purpose of the present invention the following terms are definedbelow.

The term “biopolymers” is intended to mean polymers obtained fromnatural and renewable sources.

The term “latex” as used herein is intended to mean a suspension of PHAgranules and/or particles in water. The PHA granules can be either intheir native state (amorphous), re-amorphized or re-suspended in water.The native PHA is defined as a granule of PHA, produced by bacterialfermentation, which was never precipitated, therefore itscrystallisation degree remains close to or slightly higher than what itwas in the bacteria.

The terms “granules” and/or “particles” as used herein are intended tomean spheroids shaped biopolymer segments.

The term “biomass” means the sources from which PHA is extracted. Thesesources include single-cell organisms such as bacteria or fungi andorganisms such as plants. Biomass could be wild-type organisms orgenetically manipulated species specifically designed for the productionof a specific PHA. Such modified organisms are produced by incorporatingthe genetic information (derived from bacteria which naturally producePHA) to produce one or more types of PHA.

The term “plants” as used herein is intended to mean any geneticallyengineered plant designed to produce PHA. Preferred plants includeagricultural crops such as cereal grains, oilseeds and tuber plants;more preferably, avocado, barley, beets, broad bean, buckwheat, carrot,coconut, copra, corn (maize), cottonseed, gourd, lentil, lima bean,millet, mung bean, oat, oilpalm, pea, peanut, potato, pumpkin, rapeseed(e.g., canola), tobacco, wheat, and yam. Such genetically alteredfruit-bearing plants include, but are not limited to, apple, apricot,banana, cantaloupe, cherry, grape, kumquat, lemon, lime, orange, papaya,peach, pear, pineapple, tangerine, tomato, and watermelon.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 illustrates drying of biomass with low microwave power level;

FIG. 2 illustrates drying PHBV latex with low microwave power level;

FIG. 3 illustrates drying PHBV latex with high microwave power level;

FIG. 4 illustrates unaltered biopolymer JG-B011 after 5 min. ofmicrowaving at 50% power level;

FIG. 5 illustrates unaltered biopolymer JG-B011 after 2 min. ofmicrowaving at 550 Watts;

FIG. 6 illustrates altered biomass after microwaving;

FIG. 7 illustrates the biomass microwaved during 1 min. at 1100 Watts;

FIG. 8 illustrates dried biomass after 2 min. microwaving at 1100 Watts;

FIG. 9 illustrates dried biomass after 1 min. microwaving at 550 Watts;

FIG. 10 illustrates dried biomass after 2 min. microwaving at 550 Watts;and

FIG. 11 illustrates dried biomass after 4 min. microwaving at 550 Watts.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

The present invention now will be described more fully hereinafter withreference to the accompanying drawings, in which preferred embodimentsof the invention are shown. This invention, may, however, be embodied inmany different forms and should not be construed as limited to theembodiments set forth herein; rather, these embodiments are provided sothat this disclosure will be thorough and complete, and will fullyconvey the scope of the invention to those skilled in the art.

In accordance with the present invention, there is provided a newprocess for drying biopolymers and biomass. The biopolymers are providedfrom natural sources, or derived from a fermentation process.

In accordance with the present invention, there is provided a newprocess for concentrating biopolymer solutions and biomass. Thebiopolymers are provided from natural sources, or derived from afermentation process.

The process of the invention relates to the utilisation of radiationheating techniques to dry or concentrate a biodegradable polymer or abiomass solution during its production processes. Radiation heatingtechniques may be used instead of conventional solution concentratingprocesses (centrifugation) or as a very attractive alternative tocurrent drying processes (heat treatment, vacuum dried, spray dried,freeze dried or dried on a tray in a forced air tunnel).

Illustrative of the types of biodegradable polymers which may be driedby the method of this invention are:

Biodegradable polymers comprising the following repeating unit:

wherein n is an integer from 0 up to 9 and R₁ is H, alkyl or alkenyl.Alkyl and alkenyl side chains are preferably from C₁ up to C₂₀ carbonlong and can contained heteroatoms. Biodegradable polymers can behomopolymers, with the same repeating monomer unit, and/or copolymers,with two different repeating monomer units. When biodegradable polymersare PHA polymers, they can be produced by plants or microbial organismseither naturally or through genetic engineering, or syntheticallyproduced.

Biodegradable polymers comprising two different randomly repeatingmonomer units. The two randomly repeating units have a differentstructure comprised of the general following structure:

wherein n is 0 to 9 and R₁ is H, or a C₁ to C₂₀ chain.

According to another embodiment of the present invention, types ofbiodegradable polymers which may be dried or concentrated by the methodof this invention are cellulose or modified cellulose biodegradablepolymer, starch based biodegradable polymer, PVA (polyvinylalcohol),other polyesters such as PLA, PGA and PCL and biodegradable polymer.

Illustrative of the type of biomass which may be dried by the method ofthis invention are biomasses selected from the group consisting oforganisms (plants, bacteria) natural or genetically modified.

The invention relates to a process for drying or concentrating asolution comprising any kind of biodegradable polymer that is usuallyproduced by a process comprising at least one step of drying orconcentrating the biodegradable polymer solution, wherein the solutioncomprises any kind of polar solvents. The invention also relates to aprocess for drying or concentrating a solution comprising more than onekind of biodegradable polymers.

The radiation drying can be performed with any kind of equipmentemploying radiation as a heating source. It has been found that thevolatile content of biomass or biodegradable polymer solutions can beconsiderably reduced by using radiation drying. An example of aradiation heating technique is microwave drying. During the microwavedrying process of biomass or biodegradable polymer solution, the sampleis exposed to microwave radiations. Polar solvents and water respond tomicrowave energy, so they can be eliminated from the sample. Industrialmicrowave radiation frequencies currently available are 915 MHz and 2450MHz. Some conditions may be varied during the drying process and a fewexamples of these are described below. Industrially, biodegradablepolymer solution or biomass could be disposed on a tray, on a vibratingtray and dried or concentrated by a combination of some radiationtechniques (infrared light, radio-frequency, microwaves and electricalresistance are used as generating heating sources). The thickness of thebiodegradable polymer or biomass solution layer set on the tray may beadjusted to optimize the time and the efficiency of the drying process.Radiation heating techniques could also be used as a finishing dryingprocess to remove the last remaining few percentage of moisture withoutoverheating the sample. During the drying process, dry air could also beblown into the radiation heating equipment to decrease the drying timeand to evacuate the gases (evaporated water and solvents).

Using radiation heating techniques instead of current drying processes(heat treatment, vacuum dried, spray dried, freeze dried, dried on atray in a forced air tunnel or lyophylization), considerably reduces thetime required to completely dry a sample. For instance, the minimumrequired time for a biomass lyophylization process is approximately 24hours and the drying process (vacuum oven at 45-50° C.) that takes placeduring the recovery of the PHA from a PHA-enriched solvent requiresapproximately 12 hours. Contrary to standard drying processes, radiationheating techniques allow the drying time to be reduced to a few minutesor hours, depending on the drying conditions.

This different approach for drying biodegradable polymer or biomasssolution will considerably reduce the processing time for biodegradablepolymer production. The high efficiency of radiation heating techniquesto dry biodegradable polymer or biomass solution will allow a reductionof the biodegradable polymer production costs.

The present invention will be more readily understood by referring tothe following examples which are given to illustrate the inventionrather than to limit its scope.

EXAMPLE I Drying Biomass With Low Microwave Power Level (10%)

Biomass containing PHBV (93%hydroxybutyrate-7%hydroxyvalerate) wasproduced in our laboratories according to our protocol. A Sylvaniamicrowave oven (SM81004, 1 cubic foot interior, 1600 W of consumptionpower and 1100 W of maximum microwave power) was used for the dryingprocess. 82.2 g of biomass is placed in the microwave oven, dried duringa short period of time (2 minutes) at 10% power level, removed from themicrowave, mixed and weighted. These steps are repeated until a constantsample weight is obtained. Short periods of microwave exposure arerequired to avoid the overflowing of the solution. Mixing the biomassafter each short period of microwave exposure provides a uniform dryingof the sample. In these microwave conditions, 52 minutes are required todry 82.2 g of the biomass which contains 46.5% w/w of water (FIG. 1).

EXAMPLE II Drying PHBV Latex With Low Microwave Power Level (10%)

A latex containing PHBV (93% hydroxybutyrate-7% hydroxyvalerate) wasused for the drying process. The latex contains 15% w/w of PHBV inwater.

A Sylvania microwave oven (SM81004, 1 cubic foot interior, 1600 W ofconsumption power and 1100 W of maximum microwave power) was used forthe drying process. 18.49 g of the PHBV latex is disposed in themicrowave oven, dried during a short period of time (2 minutes) at 10%power level, removed from the microwave, mixed and weighted. These stepsare repeated until a constant sample weight is obtained.

Short periods of microwave exposure are required to avoid theoverflowing of the solution. Mixing the biomass after each short periodof microwave exposure provides uniform drying of the sample. To validatethe effect of the microwave power level on the efficiency of the dryingprocess, the sample was submitted to a microwave exposure at the maximalmicrowave power level for two minutes. The high sample weight loss after46 minutes of microwave drying is explained by a short microwaveexposure of 2 minutes at the maximal power level instead of 10%. Inthese microwave conditions, 88 minutes are required to dry the PHBVlatex (FIG. 2).

In order to verify that the microwave drying process does not modify thepolymer properties, the PHBV polymer has been characterized before andafter the drying process by techniques such as, thermogravimetricanalysis (TGA), differential scanning calorimetry (DSC) and sizeexclusion chromatography (SEC). A comparison between thephysico-chemical properties of the PHBV obtained after the microwavedrying process and obtained after a speed vacuum drying process shows nodifference.

EXAMPLE III Drying PHBV Latex With High Microwave Power Level (50%)

A latex containing PHBV (93%hydroxybutyrate-7%hydroxyvalerate) was usedfor the drying process. The latex contains 15% w/w of PHBV in water.

A Sylvania microwave oven (SM81004, 1 cubic foot interior, 1600 W ofconsumption power and 1100 W of maximum microwave power) was used forthe drying process. 23.09 g of the PHBV latex is disposed in themicrowave oven, dried during a short period of time (30 seconds to 2minutes) at 50% power level, removed from the microwave, mixed andweighted. When the microwave drying process is performed at a powerlevel higher than 10%, the reduction of the microwave exposure time(minutes to seconds) is required to avoid overflowing of the solution.These steps are repeated until a constant sample weight is obtained.Mixing the biomass after each short period of microwave exposureprovides a uniform drying of the sample. In these microwave conditions,40 minutes are required to dry the PHBV latex.

In order to verify that the microwave drying process does not modify thepolymer properties, the PHBV polymer has been characterized before andafter the drying process by techniques such as, thermogravimetricanalysis (TGA), differential scanning calorimetry (DSC) and sizeexclusion chromatography (SEC). A comparison between thephysico-chemical properties of the PHBV obtained after the microwavedrying process and obtained after a speed vacuum drying process shows nodifference (FIG. 3).

The invention relates to a process for drying or concentrating asolution comprising of any kind of biodegradable polymers that areusually produced by a process comprising of at least one step of dryingthe biodegradable polymer solution or concentrating the biodegradablepolymer solution wherein the solution comprises any kind of polarsolvents or mixtures or polar solvents. The invention also relates to aprocess for drying or concentrating a solution comprising of more thanone kind of biodegradable polymers.

Appearance of a PHA derivative after microwaving under differentconditions is shown in FIGS. 4 to 11.

While the invention has been described in connection with specificembodiments thereof, it will be understood that it is capable of furthermodifications and this application is intended to cover any variations,uses, or adaptations of the invention following, in general, theprinciples of the invention and including such departures from thepresent disclosure as come within known or customary practice within theart to which the invention pertains and as may be applied to theessential features hereinbefore set forth, and as follows in the scopeof the appended claims.

1. A process for drying or concentrating with low or no degradation abiodegradable polymer in a solution or a biomass comprising submittingsaid solution or biomass to microwaves for a period of time sufficientto reduce solvent concentration from said solution or biomass inproportions of between 0.0001% to 100%.
 2. A process of claim 1, whereinsaid polymer is synthetic or natural polymer.
 3. The process of claim 1,wherein said polymer is selected from the group consisting of polyester,polyalcohol, polysaccharide, polyacid, or a mixture or a copolymerthereof.
 4. The process of claim 3, wherein said polyester is selectedfrom the group consisting of polyhydroxyalkanoate, polycaprolactone,polylactic acid, polyglycolic acid, poly(lactic-co-glycolic) acid,poly(succinic acid), or a mixture or a copolymers thereof.
 5. Theprocess of claim 3, wherein said polyalcohol is selected from poly(vinylalcohol), cellulose, or derivatives thereof.
 6. The process of claim 1,wherein said microwaves are between about 915 to 2450 MHz.
 7. Theprocess of claim 1, wherein said microwaves produced are between about100 to 1500 Watts.
 8. The process of claim 1, wherein said drying orconcentrating is performed with less than 5% degradation of saidbiopolymer.
 9. The process of claim 1, wherein said solvent is water ora polar organic solvent.
 10. The process of claim 4, wherein saidpolyhydroxyalkanoate is selected from the group consisting ofpoly-3-hydroxybutyrate, poly-3-hydroxyvalerate,poly-3-hydroxypentanoate, poly-3-hydroxyhexanoate,poly-3-hydroxyheptanoate, poly-3-hydroxyoctanoate,poly-3-hydroxynonanoate, poly-3-hydroxydecanoate,poly-3-hydroxydodecanoate, poly-4-hydroxybutyrate, and a medium chainlength PHA, or a mixture or a copolymer thereof.
 11. The process ofclaim 9, wherein said polar organic solvent is selected from the groupconsisting of alcohol, amine, amide, halogenous, cyano, aldehyde, acid,cetone, ester, thiol, and sulfoxide.
 12. The process of claim 1, whereinsaid degradation is at level between 0 to 25%.